The present invention broadly relates to a method for treating copper and copper base alloy materials to form a tarnish and oxidation resistant film. More particularly, the invention relates to an electrolyte and an electrolytic means for depositing a chromium-zinc film on a copper or copper base alloy foil.
Copper and copper base alloy foils are widely used in the printed circuit board industry. The foil is produced to a thickness of under 0.006 inches and more generally to a thickness in the range of from about 0.0002 inches (known in the art as 1/8 ounce foil) to about 0.0028 inches (known in the art as 2 oz. copper foil). The foil is produced by one of two means. "Wrought" foil is produced by mechanically reducing the thickness of a copper or copper alloy strip by a process such as rolling. "Electrodeposited" foil is produced by electrolytically depositing copper ions on a rotating cathode drum and then peeling the deposited strip from the cathode.
The foil is then bonded to a dielectric support layer forming a printed circuit board. The dielectric support layer is typically a polyimide such as Kapton manufactured by DuPont or FR-4 (a fire retardent epoxy). The copper foil layer is laminated to the dielectric carrier layer. Lamination comprises bonding the copper foil layer to the dielectric carrier layer through the use of heat and pressure. A pressure of about 300 psi, at a temperature at about 175.degree. C. for a time of up to 30 minutes will provide suitable adhesion between the layers.
To maximize adhesion, it is desirable to roughen the surface of the foil which contacts the dielectric prior to bonding. While there are a variety of techniques available to roughen or treat the foil, one exemplary technique involves the formation of a plurality of copper or copper oxide dendrites on the foil surface. U.S. Pat. Nos. 4,468,293 and 4,515,671, both to Polan et al disclose this treatment. The process produces COPPERBOND.RTM. foil (COPPERBOND.RTM. is a trademark of Olin Corporation, Stamford, Conn.).
One problem facing printed circuit board manufacturers using either electrolytic or wrought copper foils is the relative reactivity of the copper. Copper readily stains and tarnishes. Tarnishing may occur during room temperature storage of the foil or during elevated temperature lamination. The stains and tarnish are aesthetically unpleasant and may be a source of problems during the manufacture of the printed circuit board. For example, staining of copper foil prior to lamination can affect both the bond strength between the foil and the dielectric substrate and the etching characteristics of the resultant laminate.
In the past, stain resistance has been imparted to copper and copper base alloy materials by immersion in an electrolyte containing chromate ions. U.S. Pat. No. 3,625,844 to McKean, describes a method of stain-proofing copper foil involving the electrolytic treatment of the foil in a aqueous electrolyte under critical conditions of hexavalent chromium ion concentration, cathode current density, and treatment time.
U.S. Pat. No. 3,853,716 to Yates et al, discusses the McKean process and points out that it is not a completely satisfactory stain-proofing technique, due to a build-up of copper and chromium cations in the elctrolyte bath. The cations interfere with the effectiveness of the stain proofing. Yates et al attempt to overcome this problem by rendering the copper material cathodic as it passes through an aqueous electrolyte containing hexavalent chromium ion containing anions and being of sufficient alkalinity to cause precipitation of copper and chromium cations.
U.S. Pat. No. 4,387,006 to Kajiwara et al, discloses coating a copper foil with zinc chromate to prevent a reaction between the hardener added to an epoxy-glass substrate and the foil. The coating is deposited from an aqueous solution containing in excess of 1.0 g/l of both zinc and chromium (VI) ions.
Still other stain proofing techniques are illustrated in United Kingdom published patent applications 2,030,176A and 2,073,779A and U.S. Pat. No. 4,131,517 to Mutsuo et al.
Solutions of phosphoric acid, chromic acid and/or their salts have also been applied to various materials in an attempt to impart tarnish and corrosion resistance. U.S. Pat. Nos. 3,677,828, 3,716,427 and 3,764,400, all to Caule, illustrate the use of phosphoric acid solutions to improve the tarnish resistance of copper and copper-based alloys. Caule also describes in his '400 patent the use of a caustic rinse solution after application of his phosphoric acid treatment. U.S. Pat. No. 4,647,315 to Parthasarathi et al discloses a dilute aqueous chromic acid-phosphoric acid solution.
Phosphoric and/or chromic acid solutions have also been applied to zinc, zinc-coated articles and aluminum foil and articles. U.S. Pat. Nos. 2,030,601 to McDonald, 2,412,532 to Tanner, 2,418,608 to Thompson et al, 2,647,865 to Freud and 4,432,846 to Honnycutt, III illustrate some of the applications of phosphoric-chromic acid solution.
Following lamination, the anti-tarnish coating must be removed so the underlying copper foil may be etched into a desired circuit pattern. Circuit traces are patterned into the copper foil by photolithography as known in the art. The unbonded side of the copper foil is coated with a photo-sensitive chemical resist. The resist is exposed to a developer such as ultraviolet light exposed through a mask containing the desired circuit pattern. Dependent on whether the photoresist is that known in the art as "positive" resist or "negative" resist, the image may be either a desired circuit pattern, or the negative image. After exposure, the unexposed portion of the photoresist is removed by rinsing with an appropriate solvent to expose the underlying foil. The circuit board is then immersed in a suitable etchant to remove the exposed copper. After etching and rinsing, the remaining photoresist is removed by a solvent wash. The dielectric substrate is unaffected by the solvent and etchant. The substrate remains intact and the copper foil layer is patterned into a desired configuration of circuit traces.
If the anti-tarnish coating layer is not completely removed, it may interfere with the etching step during photolithography resulting in incomplete etching and the potential for an electrical short circuit. One chemical solution used to remove the anti-tarnish coating comprises 4% by volume, hydrochloric acid in water. Many of the prior art anti-tarnish coatings are not readily removed by the 4% HCl solution and require mechanical abrasion or other invasive techniques. Partial removal of the coating layer or an inordinately long process time may result.
While generally used as the etchant to remove the anti-tarnish coating from copper foil, hydrochloric acid is not desirable for environmental reasons. The chloride ions present are environmentally damaging. Regeneration of the ions into a reusable etchant or the safe disposal of the ions is an expensive proposition. A preferred solution would be to provide an anti-tarnish coating which is readily removed by a less harmful etchant.
It is known in the art that a chromium-zinc compound forms a satisfactory anti-tarnish coating for copper and copper base alloys. One such commercial coating has the composition 10 atomic % Zn; 5% Cr; 37% O; 46% C and 2% Cu. The coating is readily removed with a 4% HCl solution. However, the coating is not removable by other, more preferred etchants such as H.sub.2 SO.sub.4.